Among the many types of emergency locking retractors which have been proposed over the years are those which respond to a rapid pulling out of the belt, such as that which occurs when the occupant of the vehicle is thrown forward in a collision, an upset or an abrupt stop. This type of retractor generally includes an inertia wheel which rotates with the belt reel when the belt is pulled out relatively slowly but which lags behind the belt reel when the belt is pulled out rapidly. The inertia wheel ordinarily works in conjunction with springs which assist in maintaining a relative position between the belt reel and the inertia wheel up to some selected rate of rotation. When the selected rate of rotation is exceeded, the rotational inertia of the inertia wheel produces the lag between the inertia wheel and the belt reel. Some sort of mechanism associated with the inertia wheel responds to the lag in the rotation of the inertia wheel by locking the belt reel against rotation.
With this type of retractor the attainment of relatively great sensitivity in the mechanism, which is desirable to lock the reel against pullout of the belt when the speed at which the occupant is thrown forward is relatively low, results in the tendency for the mechanism to lock the reel when the occupant is simply pulling the belt out in the process of hooking it up. Similarly, the locking caused by a sensitive mechanism may occur during operation of a passive belt system which automatically moves the belt between a restraining configuration and releasing configuration. To overcome the inconvenience of undesired locking, it has been necessary to lessen the sensitivity of the inertia mechanism, which has meant that the retractor does not lock in the absence of a relatively rapid pullout. In general the belt-pullout sensitive types of retractors have been constructed to lock only when the passenger is thrown forward with an acceleration of about 0.7 g or higher.
The above-described problem with belt-pullout sensitive retractors has led to the development of retractors which lock not only in response to rapid pullout of the belt but to an inertia-detecting device, such as a pendulum type of device, which detects a rapid change in velocity of the vehicle, or, to be more precise, the inertia detecting device itself. Several types of so-called "double safety" locking systems which use both a belt-pullout sensitive system and an inertia detecting device to lock the reel have been proposed and commercialized. Examples of the double safety belt retractors are described and shown in U.S. Pat. Nos. 3,958,774, 4,109,881 and 3,897,024 (reissued as U.S. Pat. No. 29,594), the last of which is owned by the assignee of the present invention.
Generally, the double safety type belt retractors that have been commercialized have been judged adequate from the functional point of view. However, they have, depending on the particular one in question, a variety of disadvantages such as mechanical complexity involving a relatively large number of parts, difficulty and, therefore, high cost of assembly, unduly large size, or unduly high weight.